Thermodynamic and neutron diffraction studies of the nucleation of solid4He on graphite

The nucleation of solid layers of⁴He on the surface of exfoliated graphite (Grafoil) was investigated by thermodynamic and neutron diffraction measurements. The thermodynamic measurements consisted of recording isopycnals in the vicinity of the melting curve. The isopycnals show a rounding in comparison to the bulk ones, which can be explained by a continuous solidification of⁴He on the surface of graphite in a layer-by-layer mode in the van der Waals field of the substrate. Moreover, we found a hysteresis behavior of the isopycnals, probably due to defect states in the solidified film. The thickness of the solid film is estimated by means of a simple solidification model. The neutron diffraction results reveal microscopic details of the continuous nucleation process. It turns out that⁴He always condenses in hcp structure on the surface of graphite even in that region of the phase diagram where the equilibrium bulk phase is the bcc structure. An analysis of the linewidths and the intensities of the Bragg reflections gives information on the mean diameters, the thickness, and the orientation of the surface solid crystallites. Where comparable, the neutron diffraction data are in very good agreement with the thermodynamic results.This research was partially supported by the Bundesministerium für Forschung und Technologie (BMFT) of West Germany.